Storage tanks are routinely used to store hazardous substances, such as water products, petroleum products, chemical products and other manufacturing and industrial related liquid products. As public awareness has grown in recent years with respect to protecting the environment, various federal, state and local agencies have been directed by their respective governments to enforce laws and regulations governing the use of storage tanks that contain hazardous materials, and provide for periodic testing to detect possible leakage.
In the United States alone, there are several million underground petroleum storage tanks containing gasoline, kerosene and other fuels. In addition, there are unknown numbers of tanks containing various other hazardous substances. Although many of these tanks are made of noncorrosive fiberglass, the vast majority of underground tanks are made of steel. Consequently, the risk of corrosion failure of such a large number of tanks is significant. Undetected leaks present economic losses as well as the aforementioned environmental concerns. Thus, it is essential that such leaks be detected as soon as possible so that the tanks may be repaired or replaced. Indeed, current industry standards require that any detector employed in tank leak testing be capable of detecting leaks on the order of 0.05 gallons per hour for the more commonly encountered tanks. This capability is not easily or inexpensively achieved.
Several methods for detecting leaks in underground storage tanks are well known in the prior art. Most of these techniques use a quantitative approach to identify a leak or to determine the leak rate based on a measurement of volumetric changes of the product stored in the tank. The ability of prior art leak detection methods to accurately measure leakage is affected by certain variables such as temperature change, tank deformation, product evaporation, tank geometry and the physical characteristics of the stored product. The most significant of these factors is temperature variation, which causes dynamic expansion or contraction of the stored product on both a short-term and long-term basis. Indeed, changes in ambient temperature throughout the day are often large enough to "mask" an actual leak or cause a false leak reading where none exists. One critical problem facing tank testing entities is that changes in temperature in large underground tanks cannot accurately be correlated with volumetric changes in the underground tank fluid levels. For example, assuming that the coefficient of expansion of gasoline in a 10,000 gallon underground tank is known, a change of 0.O1.degree. F. per hour will cause for example, a 0.068 gallon change in the product volume per hour, thus offsetting or otherwise amplifying an observed leak rate. Because the coefficients of expansion for petroleum products are relatively high, conducting tank tightness tests is made difficult because rising temperatures can expand gasoline stored underground for example, at a rate equal to the leak, during the typical 5 to 10 hour test period. Problems of this type have been observed repeatedly in the field, and part of the problem is that the exact value for .beta., the coefficient of expansion, is unknown for any given stored fluid.
A number of methods for testing underground tanks are currently practiced. For example, U.S. Pat. No. 3,580,055, entitled "TANK SYSTEM TIGHTNESS TESTER", teaches that temperature and volume change should be compensated for when conducting leak tests. U.S. Pat. No. 4,954,973, discloses a method of detecting the volumetric changes the underground tank undergoes as a result of tank end deflection. The pressure against the tank ends is said to vary as a function of the specific gravity of the fluid contained. The specific gravity of the fluid to be tested is determined through the use of hydrometers. Also currently available are standardized tables which provide ".beta.", the coefficient of expansion for specific fluids at specific temperatures. These tables can also be used in conjunction with measured specific gravities or densities of the stored fluid. Thus, while the prior art recognizes that for a given fluid, .beta. will vary with temperature, and the density will also vary, both of these variables impact on the ability to measure tank leaks.
With respect to calculating .beta., the coefficient of expansion, however, a series of tables are available that can be used to determine the thermal coefficient of expansion for various petroleum products. For example, separate tables for jet fuels and kerosene, diesel fuels, heating oils and standard gasoline provide .beta. as a function of the absolute temperature. This method is not practical for use in the tank testing industry. The value of .beta. for any particular fluid varies widely as impurities are introduced into the system. Thus, no number of tables can compensate for the tolerances of .beta. at petroleum refineries, octane levels, gasoline detergents, additives, other impurities, etc. The present invention solves this problem by deriving .beta. empirically by conducting a test that measures .beta. at one or a series of temperatures. By way of example, an actual sample of gasoline is drawn from the test system known as the "Petro-tite.RTM.", at a temperature equal to the resultant value derived by circulating the gasoline throughout the tank, as disclosed in U.S. Pat. No. 3,580,055 (the '055 patent"). Thus, since the '055 patent circulates the gasoline, an average temperature can be siphoned away from the Petro-tite.RTM. tester, tested to empirically derive .beta., and then returned to the Petro-tite.RTM. closed system. The Petro-tite.RTM. closed system is assumed, for the sake of argument, to include the underground tank. Because .beta. is derived empirically, no inaccurate table need be resorted to, and gasoline brands, octanes and numerous other impurity variations can be compensated for, thus allowing a more accurate tank leak test to be conducted. U.S. Pat. No. 4,954,973 states specifically that there is a need in the industry to actually measure the coefficient of expansion "from one temperature to another to actually measure the coefficient of expansion of the fluid in the tank." The patent then states that this is impossible to do under field conditions, and goes on to rely on published .beta. values, such as those published by and available from the American Petroleum Institute, 2101 L Street, N.W. Washington, D.C. 20037. The present invention is a departure from current industry practice, which is centered around the '055 patent.
Other prior art has specifically taught the use of specific .beta. values for different types of fluids. For example, U.S. Pat. No. 4,853,694, at table 6, lists a series of temperature coefficients, listing various factors depending on the grade of fuel. It is specifically this problem that the present invention solves.